1. Field of the Invention
The present invention relates to a driving method of a piezoelectric transformer and a driving circuit for the piezoelectric transformer, more particularly to a method which is capable of driving a piezoelectric transformer with high efficiency and coping with a wide range of the power source voltage, and a circuit which drives a piezoelectric transformer with high efficiency and is operable with a wide range of the power source voltage. The driving method and the driving circuit of the present invention are suitably used for driving the piezoelectric transformer which operates, for example, a cold cathode fluorescent tube as a load.
2. Description of the Prior Art
A piezoelectric transformer is a mechanical-electrical transforming device which generates mechanical vibrations utilizing a piezoelectric effect and produces a voltage to be fetched therefrom. Since the piezoelectric transformer exhibits higher transformation efficiency compared to an electromagnetic transformer and can be fabricated to be small-sized, it has been principally used as elements which constitute an inverter and a DC-DC converter for driving a back-light of a color liquid crystal display panel (color LCD panel). A cold cathode fluorescent tube has been ordinarily used for the back-light of the LCD panel, so that to acquire a high AC voltage or a DC voltage necessary for driving the cold cathode tube, the inverter or the DC-DC converter in which the piezoelectric transformer is built is used.
Recently, the color LCD panel have used widely for not only notebook type personal computers (PCs) for but also personal digital assistances (PDAs) and monitors of car-navigation systems. When the notebook type PCs, the PDAs and the monitors of the car-navigation systems are used, the performance required for the inverter and converter of the LCD panel is high transformation efficiency and an operation in a wide voltage range so as to be able to cope with a AC power supply adapter and a battery.
As circuits for driving the piezoelectric transformers, Japanese Patent Laid-open publications No. 8-275553 (JP, 08275553, A) and No. 9-23643 (JP, 09023643, A) disclose an inverter for transforming a comparatively low DC power source voltage to a high AC voltage or a DC-DC converter for transforming the low DC power source voltage to a high DC voltage.
FIG. 1 shows a piezoelectric transformer driving circuit disclosed in JP, 08275553, A. This driving circuit comprises piezoelectric transformer 51 having primary side electrodes 511 and 512 and a pair of secondary side electrodes 513; auto-transformers 55 and 56 connected to primary side electrodes 511 and 512, respectively; switching transistors 58 and 59 for driving primary side electrodes 511 and 512 through auto-transformers 55 and 56, respectively; two-phase driving circuit 57 for complementarily driving switching transistors 58 and 59 to each other; and frequency control circuit 53 for making the amplitude peak value of load current I0 invariable. Load 52 is connected to secondary electrodes 513 of piezoelectric transformer 51.
More concretely, in piezoelectric transformer 51 which boosts (amplifies) the AC voltage inputted from the primary side circuit utilizing the piezoelectric effect and outputs the boosted AC voltage to the secondary side circuit, the secondary side output terminal of first auto-transformer 55 is connected to primary side electrode 511, and the intermediate tap of first auto-transformer 55 is connected to one output terminal 581 of first switching transistor 58. The primary side terminal of first auto-transformer 55 is connected to connection terminal 50 conducted to DC power source V.sub.DD. Control terminal 582 of first switching transistor 58 is connected to two-phase driving circuit 57, and the other output terminal 583 of first switching transistor 58 is grounded. The secondary side terminal of second auto-transformer 56 is connected to the other primary side electrode 512 of piezoelectric transformer 51, the intermediate tap of this second auto-transformer 56 is connected to one output terminal 591 of second switching transistor 59, and the primary side terminal of second auto-transformer 56 is connected to connection terminal 50. Control terminal 592 of second switching transistor 59 is connected to two-phase driving circuit 57, and the other output terminal 593 of second switching transistor 59 is grounded. Driving circuit 54 of piezoelectric transformer 51 is constituted by the primary side portion of piezoelectric transformer 51, auto-transformers 55 and 56, switching transistors 58 and 59 and two-phase driving circuit 57. It is assumed that the turn ratio of auto-transformers is N.
The driving frequency of piezoelectric transformer 51 is controlled by frequency control circuit 53 so that load current Io flowing across load 52 is invariable. The driving frequency is divided by two-phase driving circuit 57 and is subjected to a waveform shaping, whereby first and second switching transistors 58 and 59 are alternately driven.
When first switching transistor 58 is at a turning-on state and second switching transistor 59 is at a turning-off state, a current flows into the primary side of first auto-transformer 55 from DC power source V.sub.DD, and energy is charged in auto-transformer 55. Moreover, since a resonance circuit is constituted by each capacitance of second auto-transformer 56 and primary side portion of piezoelectric transformer 51 and a secondary side winding of first auto-transformer 55, half-wave sine waves are generated in primary side electrode 512 of piezoelectric transformer 51, which have a voltage amplitude obtained in such manner that the power source voltage is tripled and the tripled value is further multiplied by N+1. Similarly, when first switching transistor 58 is at the turning-off state and second switching transistor 59 is at the turning-on state, a current flows into the primary side of second auto-transformer 56 from DC power source V.sub.DD, and energy is charged in auto-transformer 56. Furthermore, another resonance circuit is constituted by each capacitance of first auto-transformer 55 and the primary side portion of piezoelectric transformer 51 and a secondary side winding of second auto-transformer 56, whereby half-wave sine waves are generated in the other primary side electrode 511 of piezoelectric transformer 51, which have a voltage amplitude obtained in such manner that the power source voltage is tripled and the tripled value is further multiplied by N+1.
Iterating such operations alternately allows piezoelectric transformer 51 to perform an AC driving at an arbitrary driving frequency. The boosting ratio of the piezoelectric transformer varies depending on its load impedance and driving frequency. Particularly, its boosting ratio becomes maximum at a resonance frequency.
Like the cold cathode fluorescent tube, in the case of a load which needs high voltages more than 1500 V.sub.rms for a lightning starting voltage and equal to about 500 V.sub.rms for a lightning voltage, since the load can not be lighted only by the boosting using the piezoelectric transformer when the power source voltage is low, the turn ratio of the auto-transformer is set to N and the driving waveform of the piezoelectric transformer is previously boosted to a value equal to (N+1) times as high as that by the auto-transformer. This boosting is called a primary boosting, and it will be possible to start an operation of the inverter by an arbitrary low power source voltage.
However, in this piezoelectric transformer driving circuit, even when the power source voltage is elevated to a value which requires no primary boosting by the auto-transformer, the auto-transformer is physically connected thereto, so that the primary boosting is continued. At this time, in order to maintain a constant output voltage to the load, the frequency control circuit makes the driving frequency to keep away from the resonance frequency, whereby it controls the piezoelectric transformer so as to lower its boost ratio. In the end, although the piezoelectric transformer driving circuit disclosed JP, 08275553, A can drive the load at a range from an arbitrary low power source to a high voltage by using the auto-transformer, it involves a problem of a reduction in driving efficiency due to a loss in the auto-transformer and a limitation to a operation voltage range due to a deformation of the driving waveform.
FIG. 2 shows a piezoelectric transformer driving circuit disclosed in JP, 09023643, A. This driving circuit comprises a piezoelectric transformer 61 having primary side electrodes 611 and 612 and secondary side electrodes 613; coil 62 provided in series between power source Vp and piezoelectric transformer 61; switching transistor 63 for driving primary side electrodes 611 and 612; an oscillator IC (integrated circuit) 65 for driving switching transistor 63; and load circuit 66 connected to secondary side electrode 613 of piezoelectric transformer 61. Here, load circuit 66 includes a voltage multiplying rectifying circuit and load 67. More concretely, one primary side celectrode 611 of piezoelectric transformer 61 is connected to one terminal 621 of coil 62 and one output terminal 631 of switching transistor 63, and the other primary electrode 612 of piezoelectric transformer 61 is grounded. The other terminal 622 of coil 62 is connected to connection terminal 64 conducted to DC power source V.sub.p, and the other output terminal 632 of switching transistor 63 is grounded. Control terminal 633 of switching transistor 63 is connected to an output terminal of oscillator IC 65.
Oscillator IC 65 outputs a control signal to allow switching transistor 63 to perform turning-on and turning-off operations. When switching transistor 63 is at a turning-on state, a current flows across coil 62 from DC power source V.sub.p, and energy is charged in coil 62. When switching transistor 63 is at a turning-off state, a resonance circuit is constituted by coil 62 and capacitance of the primary side portion of piezoelectric transformer 61, so that half-wave sine waves having a voltage amplitude higher than that of the power source are generated in one primary side electrode 611 of piezoelectric transformer 61. Switching transistor 63 performs the turning-on and turning-off operations alternately, whereby piezoelectric transformer 61 is allowed to perform an AC driving at an arbitrary driving frequency. The boost ratio of piezoelectric transformer 61 varies depending on a load impedance and the driving frequency, and particularly piezoelectric transformer 61 exhibits the maximum boost ratio and the maximum transformation efficiency at its resonance frequency.
Since the piezoelectric transformer driving circuit disclosed in JP, 09023643, A comprises no loss factor like the auto-transformer, it can drive the piezoelectric transformer with high efficiency. However, in the case of a load like the cold cathode fluorescent tube which needs high voltages more than 1500 V.sub.rms for a lightning starting voltage and equal to about 500 V.sub.rms for a lightning voltage, the boost ratio of the piezoelectric transformer lacks when the power source voltage is low, and this piezoelectric transformer driving circuit can not perform a load driving required.